The benefit of the filing date of Canadian patent application No. 2,357,430 filed on 18, Sep. 2001 is claimed.
The invention pertains to the field of imageable media and, in particular, to a multi-layer media having improved imaging performance.
Laser based digital imaging systems are commonplace in graphic arts and other imaging industries. In such systems, radiation from a laser, or an array of lasers, is focused onto an imaging medium to write an image. The radiation causes a change in the imaging medium. The emergence of higher power lasers has enabled the use of relatively insensitive media that can be handled in daylight and require less post-imaging processing to form or fix the image than earlier media.
An immediate benefit of high power imaging systems is increased imaging speed and system productivity. This is of growing importance in imaging industries. The goal of maximizing the power density of the imaging spot leads to the use of high numerical aperture (N.A.) imaging optics that conserve the brightness of the laser source. While high N.A. imaging is effective in increasing power density, systems having a high N.A. have a correspondingly narrow depth of focus. This means that the energy density of the spot will change significantly if the spot is even a few microns out of focus. This causes changes in imaged density, commonly known as imaging artefacts. Such artefacts can render the imaged media unsuitable for use.
As imaging N.A. increases with advancing technology, it becomes difficult to maintain the mechanical precision necessary to hold the system in focus. At some point, it becomes more practical to use an auto-focus system to continuously monitor and correct the focus of the imaging system rather than continue to tighten already tight mechanical tolerances.
Optical auto-focus systems are available. Such systems use a variety of techniques involving the analysis of a reflected beam to determine positional or focus information. A specific example is disclosed in Gelbart, U.S. Pat. No. 6,137,580, which is hereby incorporated herein by reference. Gelbart discloses an auto-focus system that reflects an incident beam from a surface. The Gelbart system has a position sensitive detector to receive the reflected beam and detect the position of the surface. The incident beam can be at a wavelength different from that of the imaging laser radiation. This has the advantage of separating auto-focus signals from writing signals to avoid crosstalk.
Such a system can be very effective in focusing on a wide variety of media surfaces with sufficient precision to support imaging at high N.A. For many commonly used media the focusing system works reliably with sufficient latitude to accommodate variations in the environment, imaging apparatus or the media itself.
Some lithographic plates have a substantially transparent layer coated over the active material. These transparent layers are deposited for a variety of reasons such as providing an oxygen barrier, protecting a delicate active layer, reducing the amount of ablated product or altering the properties of the plate surface. Most such media are imaged through the transparent layer, which could result in problems for an auto-focus system.
Another category of imaging media known generally as xe2x80x98thermal transfer mediaxe2x80x99 operate through a transfer of a donor material to a receiver or base, the transfer being activated by a laser imaging source. These media commonly have a donor material, which could be a dye or a pigment emulsion, adhered to a support layer. The support layer is commonly transparent which gives rise to the problematic situation where there is more than one surface capable of reflecting a focus beam.
Thermal transfer media for digital color proofing and some films are particularly susceptible to auto-focus problems. A dye-donor element is a transparent polyester film layer with a dye layer of a particular color adhered to the rear surface. The dye layer is imagewise exposed to radiation through the transparent layer, transferring the dye onto the dye-receiver base. The actual transfer process differs from medium to medium but commonly involves ablation, phase change, dye sublimation, or some sort of laser-induced transfer. Proofing media are commonly available in the base CMYK colors as well as many spot colors, which means that an auto-focus system may have to work on a great variety of different materials.
Modern imaging systems are expected to achieve a high level of efficiency and reliability. Customers have the expectation that failures will be rare since an un-noticed flaw in an imaging job can cause significant wastage and delay.
There is a general need to provide cost effective methods and apparatus for imaging which provide good reliability and efficiency.
This invention provides multi-layer media in which spurious reflections of significant intensity, which degrade auto-focus system performance, are suppressed. The invention also relates to methods for making and using such media. The invention may be applied to media of a wide range of different types. Preferred embodiments employ commonly known manufacturing steps in the fabrication of the improved media.
Further aspects of the invention and features of specific embodiments of the invention are described below.